THE BASIC CAPACITOR
If we have two metal plates close together, but separated by an insulator or dielectric (which could be air) and we apply a voltage across them, electrons are removed from one plate and applied to the other and each becomes charged. The charge held by the combination may be very large because of the concentration of the electric field between the plates. This represents a basic capacitor.
Thus, a capacitor is a device which opposes voltage change in a circuit through its capacity to store electrical energy (or charge) in the form of an electric field.